Cosmic particle accelerators

Print edition : January 18, 2019

SLAC researchers have found a new mechanism that could explain how plasma jets emerging from the center of active galaxies accelerate particles to extreme energies. Computer simulations (circled area) showed that tangled magnetic field lines create strong electric fields in the direction of the jets, leading to dense electric currents of high-energy particles streaming away from the galaxy. Photo: Greg Stewart/SLAC National Accelerator Laboratory

Most cosmic rays are protons or atomic nuclei with energies around 108 (100 million) eV. Ultra High-energy Cosmic Rays (UHECRs) have energies above 1018 (billion billion) eV and have been sporadically detected on earth since the 1960s, though their sources have remained uncertain.

Astrophysicists have theorised that the mysterious UHECRs could come from supermassive black holesand hence are called active galactic nuclei (AGNs). AGNs launch high-speed jets of plasmathat shoot millions of light years into space. A recent coincident detection of a high-energy neutrino and a gamma ray from an AGN, both of which could be explained by the acceleration of protons to very high energies, was regarded as strong evidence for AGNs as UHECR sources. But one had to rule out other viable UHECR-source candidates, such as relativistic supernovae and gamma-ray bursts. A team of researchers from the SLAC National Accelerator Laboratory in California has used simulations of particle emissions from distant active galaxies at an unprecedented scale to propose a mechanism. The results lend further credence to the idea of AGNs as UHECR sources. The results of the simulations suggest that magnetic field lines tangled like spaghetti in a bowl might be behind the most powerful particle accelerators in the universe.

“The mechanism that creates these extreme particle energies isn’t known yet,” said SLAC staff scientist Frederico Fiuza, the principal investigator of this new study that has been published in “Physical Review Letters”. “But based on our simulations, we’re able to propose a new mechanism that can potentially explain how these cosmic particle accelerators work.”

“We knew that the magnetic fields can become unstable,” said lead author Paulo Alves. “But what exactly happens when they become distorted, and could this process explain how particles gain tremendous energy in these jets? That’s what we wanted to find out.”

The researchers simulated the motions of up to 550 billion particles—a miniature version of a cosmic jet—on the Mira supercomputer at the Argonne Leadership Computing Facility at the Argonne National Laboratory in Lemont, Illinois. Then, they scaled up their results to cosmic dimensions and compared them with astrophysical observations.

The simulations showed that when the helical magnetic field is strongly distorted, the magnetic field lines become highly tangled and a large electric field is produced inside the jet. This arrangement of electric and magnetic fields can, indeed, efficiently accelerate electrons and protons to extreme energies. While high-energy electrons radiate their energy away in the form of X-rays and gamma rays, protons can escape the jet into space and reach the earth’s atmosphere as cosmic radiation.

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